Aft CG and aileron effectiveness
 

Does an excessively aft CG result in reduced ailerons effectiveness ?
Is that because the plane flies more hanging on the tail than the wings ? I have 2 plnes which suffered from excessive aft cg and had close to no aileron effectiveness and when I moved my CG forward the plane improved. Obviously it also must be the increased speed or not ? Someone knows about this relationship or tried something out ? I am talking about really excessive making the plane almost unfliable.

Thanks,
Cosmos

RE: Aft CG and aileron effectiveness
 

The aft cg should not effect the roll rate. The only way that moving the cg would effect roll rate is if you would move it up, down, or sideways. Moving it front and back would not change the moment of inertia about the roll axis, and would therefore not change the roll rate or roll acceleration (aleron effectiveness).

Speed would most likely be the cause of the decreased effectiveness.

Nmking09

RE: Aft CG and aileron effectiveness
 

It may affect roll response if there is any yaw/roll coupling and yaw stability is affected by the aft CG

RE: Aft CG and aileron effectiveness
 

It very definately can affect roll response -- an aft CG can make the plane extremely twitchy in roll response, as well as in pitch. It can't affect axial roll rate, but it can precipitate a snap, rather than an axial roll, & that can be very quick.

RE: Aft CG and aileron effectiveness
 

Outside of snap rolls following the likely stall I don't think it matters. More likely a speed thing as models with rearward CG's tend to trim to a slower level flying speed in my experience. But that's for gliders and gliding free flight models. In your case with RC the speed should be dependent on your left hand.

RE: Aft CG and aileron effectiveness
 

Ok guys, but my thinking goes like this. Aft CG forces the plane to fly nose up, tail down therefore affecting the wings angle of incidence making the airflow thru the ailerons act like they were flaps and decreasing their effectiveness. Sure the plane with speed and engine will straighten out but as my plane has no tendency to snap in a stall it just hangs tail down with almost no aileron control. Next weekend I will move the CG forward again and let you all know what happens. Just hoping someone would have a scientific aerodynamic explanation for it ;)

Regards,
Cosmos

RE: Aft CG and aileron effectiveness
 

No, aft CG doesn't make the plane fly nose-up. If you take a model, such as your basic Stik, and fly it with the CG in the normal place and get it trimmed, and then just move the balance point aft, it won't fly nose-up. You'll have to add down elevator to keep it level. It won't tend to want to fly in level flight nose-up. Pitch control should also become more sensitive, too.

Now if you mean that an aft CG means that the orignally-trimmed model will tend to pitch upward, then you're correct. But level flight will still result in a level fuselage attitude.

Most times, when a model tends to fly slowly and with the nose up, near the stall, you'll hear someone mention that the model seems "tail heavy". That's not so. That attitude of flight usually means too little power, which results in the wallowing flight.

RE: Aft CG and aileron effectiveness
 

Actually, in most cases, moving the CG rearward results in the plane flying level in a slightly more nose DOWN attitude. This is because moving the CG rearward reduces the negative (downward) lift required of the stabilizer, which in turn reduces the lift required of the main wing to maintain level flight. Therefore, you need less angle of attack to fly level.


On the original question, don't confuse equilibrium issues with stability issues. Aileron effectiveness and roll rate are affected by aileron size & position and the plane's lateral moment of inertia. The dynamic stability of the plane will be reduced with an aft CG, which of course gives you the "twitchy" flying plane. The fact that the plane, at a point, would rather fly tail first and is hard to control makes you use all control surfaces more, but that is not the same as saying the aileron effectiveness has changed- it does not. If you are more liberal with the definition of "effectiveness" than the normal definition used in the aeronautical world, than yeah, when the plane is out of control because it's tail heavy, nothing, including the ailerons, is going to be "effective" in resolving that issue.

RE: Aft CG and aileron effectiveness
 

A little additional data. The rolling moment from ailerons is pretty much constant up to the start of the initial fall off of the lift curve - around 10 to 12 degrees or so. It applies to both + and - control defections. So regardless of the CG location the moment provided by the ailerons is going to be fairly constant (assume same deflections and sizes) for almost all normal flying conditions. There might be exceptions but in general it is true.

You probably have seen drawings of airplanes with a 3 dimensional axis system put through it. Without any dynamic or inertial coupling the ailerons only effect roll, elevator pitch and rudder yaw. Those are the 0 angle of attack forces from control deflections. Moving the location of the center of the axis system doesn't change the basic moments from the control deflections.

Letting the airplane actually fly at a angle of attack doesn't change them much either. When maneuvering there are inertial and dynamic effects that enter into it but based on a lot of experience there isn't the kind of variation that you noted.

By the way, being able to tell the difference in roll rates is a tough thing to do based on just observations from different flying sessions. Of course I fly so many different airplanes each time out I can barely remember anything about how they fly so I am probably putting my inability on you!!. The thing to do would be to get a stop watch and time several rolls to get the roll rate. Then you could determine if what you "felt" was indeed true.

RE: Aft CG and aileron effectiveness
 

Ok, after further review I have come to the conclusion that my previous post was wrong. You can indeed increase your roll rate by simply moving the CG back. On reason is that if you have a forward CG It causes more down-force to be on the tail, causing you to fly at a higher aoa, so when you roll you are not actually doing a true axial roll but rather something that resembles more of a tight barrel roll or a slide.

Another reason is that when your aircraft becomes more sensitive in pitch that means that the forces on the tail have more of an effect on the aircraft. This not only includes pitching forces due to elevator deflection, but pitching forces due to a change in downwash angles. Downwash is the effect that the airflow around the wing has on the tail and is related to the CL of the wing

This means that if you increase the CL on one side of the wing, and decrease the CL on the other side of the wing (which is what happens when you deflect ailerons), you in essence cause not only a rolling moment on the wing, but on the tail as well. This is because the downwash angle on one side of the airplane is different that the other side. The effect this has on the overall roll rate of the aircraft increases as the CG moves aft because pitch rate increases as CG moves aft. You would most likely see this effect on aircraft with large ailerons. This is definitely not a first order effect and is highly dependent on many variables.

Downwash angle change is the reason that some aircraft pitch up when flaps are deployed. The wing CL increases, the tail sees a higher downwash angle, thereby decreasing the effective incidence of the stab. Imagine if one side had flaps and the other had spoilers. You can imagine that this would cause an un-simmetrical pitching moment, thereby causing a rolling moment.

This is about all the explanation I have for now.

Nmking09

RE: Aft CG and aileron effectiveness
 

A forwarg CG will not only cause a higher AOA, it will also increase the wing loading.

Ted

RE: Aft CG and aileron effectiveness
 

How is this possible? ----Either area you've touched on.

Without an increase (or decrease) in overall weight of the A/C wing loading is unchanged. CG has nothing to do with the loading calculations.

If you have an explanation I'll be more than happy to consider it.

RE: Aft CG and aileron effectiveness
 

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??????????

RE: Aft CG and aileron effectiveness
 

Tall Paul...

I think what he's saying is that a forward CG requires a higher tail down force (negative) to maintain a level pitch attitude. The sum of the increased tail down force AND gravity equal an airplane that has a greater total downward force exerted on it. It would seem reasonable to view this as an increase in 'effective" wing loading. (if there is such a term)

Because of the increased downward forces (gravity + TDF), the wing must now operate at an increased AOA, OR...a higher airspeed to provide sufficient lift for level flight.

This is precisely why a nose heavy airplane has a longer take off roll, increased landing speeds, and believe it or not...an increased stalling speed. It is also the reason why all other things being equal, an aircraft with an aft CG is faster than one with a forward CG.

Better?

RE: Aft CG and aileron effectiveness
 

Frankly no.

RE: Aft CG and aileron effectiveness
 

Do elaborate.

RE: Aft CG and aileron effectiveness
 

Thanks RaceCity, exactly! (you beat me to the punch)

Yes fellas, it sounds strange but it is ABSOLUTLY TRUE !

The plane does not weigh any more, but the downforce on the tail required to put the plane in equilibrium is in fact acting on the wing while flying.

Heres a good way to 'see' it. (oversimplified)

Put your favorite 10 lbs plane on the balance stand, with the stand sitting on a scale, plane balances straight and level.
Now, move the battery way forward, and the nose pitches down. Now, to get it back to straight and level, we need to add -say 6 oz to the elevator to get it back level. The scale reads 10 lbs, 6 oz. Remember, the 6 oz of lead ? That 6 oz is the force required to put the plane back to a level flying state. 6 oz is 6 oz, weather its a 6 oz lead weight on the elevator, or deflected air creating the 6 oz downforce.

Ted

RE: Aft CG and aileron effectiveness
 

Thanks for your explanation---------However, try this logic: The 10# A/C is flying dead level (lift equals weight) your scales would read zero----if we could weigh it in flight.

I see what you are driving at but that is not the classic way you calculate wing loading. For models it would be wing area (plan view) in square inches over 144 giving an answer in square feet.
The weight of the A/C in ounces over the calculated square feet gives you the ounce loading per square foot. In flight the A/C will encounter 'G' forces------these do not alter the measurement of wing loading as used for a measurement of an A/C's specs..

No need for further explanations-------we are simply on different wave lengths.

RE: Aft CG and aileron effectiveness
 

The increase in "effective wing loading" due to a forward c.g. is ignored.
If that's all it takes to collapse the structure, then back to Stucture 101.
Planes are built for manuvering loads, not level flight cruise loads.

RE: Aft CG and aileron effectiveness
 

TallPaul,

Thank you ----

THAT adds credence to the point I was trying to convey. Being a non-engineering type (but lots of flight time) I couldn't express it with any authority, or with a great deal of clarity.

RE: Aft CG and aileron effectiveness
 

My statement has absolutely nothing to do with the classical definition of wing loading, but more of a practical reference.

Tall Paul, there's more than a few full scale pilots that most definitely pay attention to the "effective wing loading due to forward cg"! (negligible or not, something about saving fuel appeals to them.)

Onewasp - Yeah, weigh your example plane in flight - 0 lbs, lift=weight, now shift the cg forward, add elevator untill the wing reaches the same AOA as before the cg shift, and I guarantee lift won't equal weight anymore. Increase the aoa or airspeed, and then lift will equal weight. Oh, and by the way, I didnt appreciate the wavelength comment.

RE: Aft CG and aileron effectiveness
 

Wrong again!

RE: Aft CG and aileron effectiveness
 

Whatever....................

RE: Aft CG and aileron effectiveness
 

Guys, I hate to break it to you but F2G is right about the tail load. I know it isn't normally dealt with in the classic wing loading equation but that does not mean it's not there. There's lots of simiilar anomalies in our other classic calculations. For the most part it is often a minor issue but that does not mean it's not there.

My free flight models often use this charactaristic to GAIN lift by using the tails positive lift in conjunction with a rearward CG to "get something for nothing" and it's the reason why the FAI and AMA free flight events include the tail area where a lifting surface area rule is being enforced.

RE: Aft CG and aileron effectiveness
 

Obviously you are right and I am wrong!

"Guys, I hate to break it to you but F2G is right about the tail load. I know it isn't normally dealt with in the classic wing loading equation but that does not mean it's not there. There's lots of simiilar anomalies in our other classic calculations. For the most part it is often a minor issue but that does not mean it's not there."

BMatthews
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